FIG. 7 shows a conventional thermal head. The thermal head B includes an insulating substrate 91, and a glaze layer 92 made of e.g. glass and formed on the substrate. An electrode 93 and a heat-producing resistor 95 are formed on the glaze layer 92. A protective layer 96 for covering the heat-producing resistor 95 and the electrode 93 are formed by printing and baking amorphous glass.
To perform printing using the thermal head B, a platen roller P is arranged to face the heat-producing resistor 95. In printing, with thermal recording paper S as the printing medium pressed against the protective layer 96 by the platen roller P, the thermal recording paper S is moved in a secondary scanning direction through a distance corresponding to one line, for example. Then, the heat generated at the heat-producing resistor 95 is transferred to the thermal recording paper S via the protective layer 96 to change the color of the paper at that portion, whereby printing is performed. Thereafter, by alternately repeating the movement of the recording paper S line by line and the printing process by the thermal head B, printing is performed with respect to the entire recording paper S.
In the printing process using a thermal head, the so-called sticking may occur. The sticking is a phenomenon that the thermal recording paper adheres to the obverse surface of the protective layer so that the transfer of the thermal recording paper becomes irregular. Due to the sticking, a print failure such as the appearance of a white line on the thermal recording paper may occur.
As a method to prevent the sticking, it may be considered to reduce the frictional resistance between the thermal recording paper and the protective layer by smoothing the obverse surface of the protective layer. Therefore, in the conventional thermal head B, the protective layer 96 to be pressed against the recording paper in the printing process is made of amorphous glass to suppress the sticking, because amorphous glass is excellent in smoothness of the surface.
FIG. 8 shows another example of conventional thermal head which utilizes amorphous glass to suppress the sticking. As shown in the figure, this thermal head includes a protective layer 96 having a double-layer structure comprising the lamination of different kinds of layers, i.e., a first protective layer 96A and a second protective layer 96B. In this thermal head B′, of the two layers, the first protective layer 96A which is on the lower side is made of crystallized glass having excellent wear resistance, whereas the second protective layer 96B on the upper side is made of amorphous glass having excellent smoothness. In this way, in the thermal head B′ shown in FIG. 8, the first protective layer 96A having excellent wear resistance is provided under the second protective layer 96B to be pressed against the recording paper S. Therefore, the wear resistance of the thermal head is enhanced as compared with the thermal head B shown in FIG. 7.
In the printing process using a thermal head, the adhesive force of the thermal recording paper to the protective layer is relatively large, because the thermal recording paper is pressed against the protective layer during when it is transferred. Further, the component of the protective layer or of the thermal recording paper may soften due to the heat produced at the heat-producing resistor. In such a case, the adhesive force further increases.
The removal of the thermal recording paper from the protective layer can be facilitated by smoothing the obverse surface of the protective layer and reducing the frictional resistance to as small as possible. However, even when the frictional resistance at the obverse surface of the protective layer is reduced, the recording paper may not be reliably removed from the protective layer in the case where the recording paper adheres to the protective layer not only due to the pressing force by the platen roller but also due to the softening of the component of the protective layer or of the thermal recording paper caused by the heat production at the heat-producing resistor. Therefore, the sticking cannot be sufficiently prevented by the conventional thermal heads B and B′ in which only smoothing of the obverse surface is attempted by making the protective layer 96 or the second protective layer 96B to be pressed against the thermal recording paper by utilizing amorphous glass.
As another method to prevent sticking, it may be considered to reduce the pressing force for pressing the thermal recording paper to the protective layer. According to this method, however, heat cannot be sufficiently transferred to the thermal recording paper, which may cause problems such as degradation of printing quality.
Patent Document 1: JP-A-S63-74658
Patent Document 2: JP-A-2001-47652